This invention relates to a process for preparing polyisocyanates that can be used as a curing agent for two-pack urethane resins usable in the fields of paint, adhesives and molding materials.
More specifically, this invention relates to a process for preparing polyisocyanates of the isocyanurate type which is derived from a diisocyanate compound mainly composed of hexamethylene diisocyanate and which can be used as a curing agent for two-pack urethane resins.
The isocyanurate-type polyisocyanates, which are derived from hexamethylene diisocyanate through isocyanuration reaction, are important since they are more durable curing agents for two-pack urethane resins than conventional biuret-type or adduct-type polyisocyanates due to the high chemical stability of the isocyanurate ring contained therein.
Also, since the isocyanurate-type polyisocyanates have in the molecules no such bonds as urethane bonds which cause hydrogen bonding, they are considered good curing agents for two-pack urethane resins used in paints and adhesives due to their low viscosity and good dissolving power in various organic solvents.
However, in the process for preparing isocyanurate-type polyisocyanates with an isocyanuration catalyst, hexamethylene diisocyanate has different properties than those of aromatic diisocyanate compounds such as tolylene diisocyanate, indicating high selectivity for the catalyst and tending to involve side reactions, e.g., carbodiimide formation, that readily take place causing coloration or the like. Therefore, isocyanuration catalysts having particularly high performance are required in this process.
Furthermore, the isocyanurate-type polyisocyanates derived from hexamethylene diisocyanate when used as a curing agent for two-pack urethane resins are known to have somewhat poor compatibility with acrylic polyols which are widely used as a major component of the curing agent. Accordingly, to prepare isocyanurate-type polyisocyanates having improved compatibility with the acrylic polyols, they must be polyisocyanates which have a low molecular weight and in which the content of the compounds containing only a single isocyanurate ring (hereafter, referred to as "mononuclear" polyisocyanates or compounds) is high. That is why isocyanuration catalysts having improved performance are required.
However, since hexamethylene diisocyanate has high selectivity for catalysts, conventional isocyanuration catalysts (i.e., various tertiary amine compounds and phosphines) that are efficient for aromatic diisocyanate compounds such as tolylene diisocyanate cannot be used in the isocyanuration of hexamethylene diisocyanate. Accordingly, there has been an ongoing need to develop other efficient isocyanuration catalysts useful for the purpose.
As the results of extensive studies conducted recently on the catalysts useful in the isocyanuration of hexamethylene diisocyanate, N-(2-hydroxyalkyl)-quaternary ammonium aliphatic carboxylate was proposed as a catalyst with high catalytic activity. Japanese Patent Publication (Kokai) No. 55-143978 discloses use of the catalyst in amounts of from 200 to 1,000 ppm to prepare isocyanurate-type polyisocyanates. Although this process using a quaternary ammonium compound catalyst is more advanced when compared as the preceding proposals, the isocyanuration reaction performed using hexamethylene diisocyanate alone causes coloration of the resin due to accompanied side reactions, failing to provide isocyanurate-type polyisocyanates having a light color and acceptable quality.
Very recently, the present inventors have found in their study that N-(2-hydroxyalkyl)-quaternary ammonium aromatic carboxylate is an efficient isocyanuration catalyst with high activity (Japanese Patent Publication (Kokai) No. 60-181078). However, the process of the preceding proposals proved inadequate to suppress the coloration which occurred during the isocyanuration reaction when hexamethylene diisocyanate was used alone. Thus far, light color, high-quality isocyanurate-type polyisocyanates with a high content of a mononuclear polyisocyanate compound have been difficult to prepare at comparatively low conversions.
As previously mentioned, the conventional processes proved insufficient for preparing light color, high-quality isocyanurate-type polyisocyanates using hexamethylene diisocyanate alone.